1. Field of the Invention
This invention relates to a method and apparatus for extruding polymer, ceramic, composite, and other materials that exhibit anisotropic properties due to molecular or fibril orientation along a single axis as a result of the extrusion process. More particularly, the invention relates to an improved extrusion method and apparatus that provides a multi-axis orientation of molecules or fibrils in the extruded material.
2. Description of the Prior Art
Certain polymers (important examples are liquid crystal polymers), composites, and ceramics are comprised of molecules and/or fibrils that become more or less orientated along a single axis when extruded through fixed die openings. The resultant extruded films, sheets, or parts have physical thermal, chemical and electrical properties that are anisotropic; i.e., one value in the direction of orientation and another value in a direction perpendicular to the direction of orientation. FIG. 1 is a schematic view of a flat sheet extruded with a conventional die assembly. As illustrated by the dashed lines, elongated elements (e.g. fibrils in a liquid crystal polymer) are oriented in the flow direction (indicated by the arrow) through the die assembly.
The prior art has proposed applying a shearing force to the surfaces of the material in the liquid state as part of the extrusion process. This force produces a multi-axial orientation and a resultant extruded material that is less anisotropic than that produced with fixed extrusion dies. These proposals include the following U.S. patents.
U.S. Pat. No. 4,450,131 discloses a die assembly for extruding a liquid crystal polymer with two rotating cylindrical die members of different diameter which are situated coaxially;
U.S. Pat. No. 4,496,413 discloses a die assembly with a cylindrical tubular die member surrounding a solid counter-rotating core; and
U.S. Pat. No. 4,420,451 discloses a method for film extrusion comprising rotary die parts. The die parts comprise different diameters and are arranged surrounding each other.
In these prior art devices the material is extruded in the form of a tube. To form a sheet material the tube has to be cut or folded upon itself, which is disadvantageous.